Field:
The present invention relates to micro-optical electromechanical devices and especially to a micro-optical electromechanical device and a method for manufacturing a micro-optical electromechanical device, as defined in the preambles of the independent claims.
Description of the Related Art:
Mirrors for reflecting a beam of light have been developed based on MEMS technologies. In a scanning mirror, the direction of reflection can be changed as a function of time. A scanning mirror can direct the light beam over a range of directions in one or two dimensions, and it may also be used to collect light from a range of directions with good angular accuracy and resolution. Scanning operation over an angular range is obtained by tilting the mirror to an angle and varying this angle as a function of time. Often this is done in a periodical or oscillating manner. To obtain high scanning angular amplitude the mirror may be operated in a resonant mode. A resonant mirror may be supported by springs. The rotational inertia of the mirror, and the spring forces are the elements forming a mechanical resonator. There are several applications for such scanning mirrors, e.g. code scanners, scanning displays and laser ranging and imaging sensors (Lidars).
It has been recognized that a silicon wafer is an excellent raw material for a scanning mirror since it is available in a highly polished form, it has low weight and excellent springs can be produced to the same silicon wafer to enable operation in a resonant mode. The earliest silicon scanning mirrors were utilizing electromagnetic forces to excite the scanning motion. They had current loops on the moving part, and permanent magnets attached to the body of the mirror device. Later mirrors with electrostatic actuation were developed, and they didn't require large size permanent magnets. U.S. Pat. No. 8,305,670 is an example of the state of the art of electrostatically actuated scanning mirrors.
FIGS. 1a to 1c illustrate basic principles of a conventional electrostatically driven mirror. FIG. 1a is a cross section of the device with the mirror 12 in rest position. FIG. 1b shows the cross section when the mirror 12 is tilted and FIG. 1c shows the top view of the device. To the mirror 12 moving comb electrodes 14 are attached. First stationary comb electrodes 15 and optionally second stationary comb electrodes 16 are attached to the non-moving body of the mirror device (not shown). The mirror is suspended with torsion springs 17 to the mirror body.
Voltages are applied between moving and stationary comb electrodes and controlled so that a force is created in a vertical direction causing the mirror to tilt. The force is proportional to the second power of the applied voltage and to the second power of the inverse of the distance between electrodes. Voltages up to 200 V have been used to drive such electrostatic actuating transducers.
In addition to the high voltages needed, the electrostatic actuator has other limitations when used in scanning mirrors. For a high force the distance between electrodes should be made as small as possible. In practice this distance is limited by the thickness of the material and the maximum aspect ratio of the etching method used to manufacture the electrodes. In practice the maximum practical aspect ratio is less than 50:1. The force is maximized for a given aspect ratio if the distance between the electrodes is made as small as possible. For example, if this distance was chosen to be 2 μm, the height of the electrode would be 100 μm or less with a realistic etching process.
The required size and the required scanning angle of the mirror depend on the application. A mirror for a lidar should be quite large for efficient light collection and so is the required angle: 7 mm diameter and +−15 degrees scanning angle have been presented as target values for an automotive lidar in the EU-funded MiniFaros-project. With this diameter and angle, the edge of the mirror will be displaced by +/−900 μm from the rest position. If the height of the electrode is 100 μm, the electrode overlap will be zero for roughly 90% of the time during which no force is produced. Electrostatic drive is thus very inefficient for large amplitude excitation.
Due to manufacturing reasons and due to the large amplitude required, the electrostatic transducers are conventionally located at the periphery of the mirror, such as in U.S. Pat. No. 8,305,670. This increases the size of the mirror device and also the rotational inertia of the mirror, which must be compensated by higher force.